improve base instruction weight benchmark

The former instr_i64const benchmark has had the problem in that it was too simple to optimize for the new Wasmi (register), resulting in an artificial 20x speedup since Wasmi (register) mostly optimized the whole function body away.
The new benchmark body still makes use of the new Wasmi (register) optimizations but in a way that no computation is lost and thus the benchmark is more aligned to real world base weights for both Wasmi (stack) and Wasmi (register) executors.